(1) Field of the Invention
The present invention relates to the field of electrical machines. It relates to a reversible electrical machine, i.e. an electrical machine that may act as a motor or as a generator. The invention relates more particularly to a reversible electrical machine for a rotary wing aircraft, and also to a power plant for such an aircraft.
(2) Description of Related Art
An electrical machine is said to be “reversible” when it makes it possible both to transform electrical energy into mechanical energy and also to transform mechanical energy into electrical energy.
An electrical machine transforming electrical energy into mechanical energy is known as a motor. An electrical machine transforming mechanical energy into electrical energy is known as a generator. Among generators, it is possible to distinguish between alternators that deliver electricity in the form of alternating current (AC) and dynamos that deliver electricity in the form of direct current (DC).
Any electrical machine may be reversible, i.e. it may be both a motor and a generator, with the distinction between the motor and generator functions being determined solely relative to the purpose for which the electrical machine is used. The term “motor-generator” is also used if both of those functions are available from the electrical machine.
The motors commonly in use may be rotary, i.e. they produce angular movement and/or torque, or else they may be linear, i.e. they produce linear movement and/or force.
In contrast, generators are almost always rotary.
A rotary electrical machine is an electromechanical device having at least one stator that is stationary and at least one rotor that rotates relative to the stator. The rotation of the rotor is generated by the interaction between two magnetic fields attached respectively to the stator and to the rotor, thereby creating a magnetic torque on the rotor. The terms “stator magnetic field” and “rotor magnetic field” may then be used.
The description below is restricted to rotary electrical machines, so the term “electrical machine” is used instead of “rotary electrical machine” for short. Likewise, the term “electrical machine” designates a rotary electrical motor, and the term “generator” designates a rotary electrical generator.
The various technologies used for electrical machines differ essentially in the way in which these stator and rotor magnetic fields are generated.
For example, in a DC electric motor, the stator has magnetic elements that may be permanent magnets or that may be non-permanent magnets, more commonly referred to as electromagnets, and generally constituted by windings of electrical conductors, such as one or more coils powered with DC. Either way, a steady stator magnetic field is thus created. In contrast, the rotor has non-permanent magnets constituted by a set of coils creating a rotor magnetic field when they carry DC. During rotation of the rotor, a rotary commutator serves to reverse the direction of DC flow through the coils of the rotor at least once per revolution, thereby changing the direction of the rotor magnetic field.
Thus, an offset between the stator and rotor magnetic fields gives rise to magnetic torque on the rotor, for example with a north pole of the stator repelling a north pole of the rotor and attracting a south pole of the rotor. Consequently, the rotor is caused to rotate relative to the stator.
A main drawback of such a DC electrical machine lies in the electrical contacts needed between the rotor coils and the rotary commutator. These contacts, e.g. obtained by brushes, can give rise to electric arcs leading in particular to wear and to interference, and consequently requiring maintenance to be performed on the electrical machine more frequently. In addition, electric motors of that type are not suitable for high speeds of rotation and they consume energy in friction, thereby degrading efficiency. Finally, they can be complex to make.
That drawback has been eliminated by so-called “brushless” motor technology.
The rotor of such a motor has one or more permanent magnets, while the stator has a plurality of coils constituting non-permanent magnets. Such a motor may also have means for determining the position of the rotor, e.g. by using a sensor, together with an electronic control system for switching electric current. AC is then passed through the various stator coils. The electronic control system thus serves to determine the orientation and the direction of the stator magnetic field relative to the rotor magnetic field and consequently causes the rotor to rotate relative to the stator, with a rotary stator field pulling the rotor field into synchronization.
Furthermore, one or more coils of the stator may be grouped together in order to form different phases of the stator, each phase having the same offset relative to the other phases. Each phase thus generates a stator magnetic field, each stator magnetic field being likewise offset relative to the other stator magnetic field.
Among AC electrical machines, a distinction may be drawn between electrical machines that are synchronous and those that are asynchronous.
Synchronous electrical machines, a category in which brushless motors belong, have a rotor with one or more permanent magnets and a stator with one or more coils, thus serving to form one or more phases. When they convey one or more phases of AC, the coils of the stator create one or more rotating stator magnetic fields that pull the rotor magnetic field into synchronization with the synchronous frequency of the machine, and consequently entrain rotation of the rotor.
Conversely, rotation of the rotor, e.g. generated by external mechanical power, creates a rotating rotor magnetic field, thereby causing one or more rotating stator magnetic fields to be created, and consequently causing one or more AC phases to appear and flow in the coils of the stator.
The permanent magnets of the rotor may be replaced by DC-powered coils, thereby constituting non-permanent magnets, and thus creating an equivalent rotor magnetic field. The DC may be delivered by a source of electricity, such as a battery or a capacitor.
The frequency of rotation of the rotor of a synchronous electric motor is proportional to the frequency of the AC applied to the stator. Likewise, the frequency of the AC generated by a synchronous electric generator is proportional to the frequency of rotation of the rotor. A synchronous machine is often used as a generator, e.g. as an alternator in power stations.
Asynchronous electrical machines have a rotor with one or more short-circuited coils and a stator having a plurality of coils constituting non-permanent magnets. When the coils of the stator carry AC, they create one or more rotating stator magnetic fields that cause rotor electric current to appear in the rotor coils, thereby generating magnetic torque on the rotor, and consequently causing the rotor to rotate relative to the stator.
Conversely, rotation of the rotor as generated by external mechanical power will lead to the appearance of AC flowing in the coils of the stator. For this purpose, it is necessary to connect the electrical machine to an electricity network, e.g. including at least one converter and a battery, in order to supply it with the reactive energy needed to enable it to operate in generator mode.
Although the frequency of rotation of the stator magnetic field is proportional to the frequency of the AC carried by the coils of the stator, the frequency of the rotation of the rotor of an asynchronous electric motor is not necessarily proportional to this frequency of the AC, and a slip speed may appear between the rotor and the stator magnetic field. Likewise, the frequency of the AC generated in an asynchronous electric generator is not necessarily proportional to the frequency of rotation of the rotor.
For a long time, asynchronous machines were used only as electric motors, e.g. in transport for propelling ships and trains, and also in industry for machine tools. By using power electronics, such electrical machines can nowadays also be used as electricity generators, e.g. in wind turbines.
Whatever the type of reversible electrical machine, the permanent or non-permanent magnets may be oriented either radially or else axially relative to the axis of rotation of the electrical machine. These various orientations of the magnets enable the magnetic flux flowing in the electrical machine to be oriented either radially or axially. On a given machine, it is also possible to use some magnets that are oriented radially and others that are oriented axially.
Electrical machines having permanent magnets on the rotor provide greater performance because of their high efficiencies in motor mode, making it possible to have high power per unit weight and because of the high level of magnetic torque that is generated on the rotor by the permanent magnets. Furthermore, since they do not make use of brushes, friction is kept down, thereby contributing to improving the performance of such electrical machines, in particular at high speeds of use, and also contributing to reducing the frequency with which maintenance is required.
In contrast, in an electrical machine that is reversible, in order to have maximum efficiency, the mechanical power delivered by the rotor in motor mode must be equal to the electrical power that can be generated on the stator in generator mode. If all of the power in generator mode is not used, that leads to high levels of losses by the Joule effect and to a degradation in terms of efficiency and in terms of the power per unit weight of the machine, in particular because of the need to add a large system for dissipating the heat generated by the Joule effect.
Furthermore, a short circuit on one or more coils of the stator, and consequently on one or more phases of the stator, can give rise to a high level of opposing torque that can lead to a rapid loss in the speed of rotation of a reversible electrical machine. This braking of the rotation of the electrical machine can have severe consequences, e.g. on a transmission member that is mechanically coupled to the outlet shaft of the electrical machine. Finally, during rotation of the rotor, a short circuit on one or more coils of the stator can give rise to high induced currents in each short-circuited coil, and that can lead to local heating of the coil that might cause a fire.
Although reversible, that type of electrical machine therefore does not make it possible to achieve performance levels that are equivalent both in motor mode and in generator mode if there is unbalance in terms of demand between those two modes. Consequently, a sufficiently high safety level is not achievable for incorporating such a reversible electrical machine with permanent magnets in a rotary wing aircraft, for example.
Electrical machines with coils in the rotor make it possible to eliminate those drawbacks, i.e. the resisting torque that breaks the rotor and the risk of fire due to a short circuit in one or more coils of the stator. Such electrical machines have performance levels that are equivalent in motor mode and in generator mode and they enable the currents that are applied to the coils of the rotor in generator mode to be adjusted. Nevertheless, such electrical machines present lower performance in motor mode than can be achieved in machines using permanent magnets. In order to obtain a level of performance that is equivalent to an electrical machine having permanent magnets, in particular in terms of torque in motor mode, such electrical machines would need to be of larger dimensions, i.e. they need to have volume and weight that are greater.
Furthermore, the use of such reversible electrical machines on board vehicles, and in particular on board rotary wing aircraft, is being developed in order to provide hybrid power plants that make use of two types of energy: both heat energy from fuel and electrical energy. Nevertheless, various constraints of such electrical machines limit their applications.
A distinction may be drawn in particular between two types of hybrid power plant used on rotary wing aircraft. Firstly, there is parallel hybridization in which an electric motor is connected to the main rotor of the aircraft, e.g. via a main gearbox (MGB), in order to deliver mechanical power to the main rotor and in order to recover mechanical power from the rotor, in particular during very specific stages of flight. There is also micro-hybridization in which an electric motor is connected to a turboshaft engine via a compressor of the engine, e.g. in order to deliver mechanical power to the engine and in order to recover mechanical power from the engine during specific stages of operation, other than starting the engine.
Firstly, for a rotary wing aircraft, the reversible electrical machine may be connected mechanically to the MGB or to the compressor of a turboshaft engine, and the torque requirement of the electrical machine in motor mode is large. However, as mentioned above, this torque requirement is prejudicial to the performance of such an electrical machine in generator mode since the electrical power demand in generator mode is much less than the mechanical power demand in motor mode, or else it is necessary to use electrical machines that are heavy and bulky, and in particular that include large cooling systems. It is also possible to use two electrical machines, a first electrical machine that performs the motor function only and a second electrical machine that performs the generator function.
Furthermore, in the event of a short circuit on at least one coil of one of those electrical machines, high levels of damage can be generated, in particular in the MGB of the aircraft and in the environment surrounding the electrical machine. In order to avoid such damage, one solution is to use a fuse section of small diameter on the connection between the electrical machine and the MGB, for example. However the dimensioning of that section is such that its breaking torque must be less than the resisting torque from the electrical machine as generated in the event of a short circuit and greater than the torque from the electrical machine when operating in motor mode. The energy needed for breaking that fuse section is therefore very high. Consequently, such a fuse section is difficult to install, since the resisting torque from the electrical machine and the torque delivered by the electrical machine in motor mode may be equivalent. It is nevertheless possible to overdimension the electrical machine so as to have a resisting torque from the electrical machine in the event of a short circuit on at least one coil that is greater than the torque delivered by the electrical machine in motor mode. However, such a machine then has dimensions and weight that are not favorable for use on board a rotary wing aircraft.
Another known solution is to use a disengageable connection such as a freewheel or overrunning clutch between the electrical machine and the MGB. That type of connection makes it possible to avoid damaging the MGB in the event of such a short circuit, but it prevents the electrical machine from being used in reversible manner, thereby losing its generator function.
Document EP 1 324 893 described an electrical machine having two electrical assemblies, each having a rotor and a stator together with a freewheel between the two rotors. In addition, a first assembly of that electrical machine may be connected via the first rotor to the transmission shaft of an engine of a self-propelled vehicle, while the second assembly may be connected via the second rotor to a clutch that is connected to the gearbox of the vehicle. The freewheel thus serves to associate the two rotors in one direction of rotation and to dissociate them in the opposite direction.
The first electrical assembly is capable of starting the engine. When the engine is driving the gearbox, both electrical assemblies are also driven by the engine via the freewheel. They can then operate in generator mode in order to deliver electricity. In electrical propulsion mode, the engine is stopped and the second electrical assembly acts on its own to drive the gearbox, the freewheel ensuring that the second electrical assembly and the engine are not driven by the second electrical assembly. Finally, e.g. when descending, the freewheel ensures dissociation between the first and second rotors, it being possible for the engine then to be idling. The first electrical assembly can then be used as a generator, with the second electrical assembly being used either as a generator or else as a brake.